This invention relates generally to techniques for making underwater connection of offshore pipelines, and more particularly relates to the installation of a prefabricated spool into position between two underwater pipelines.
When a pipeline break occurs on an offshore pipeline, the broken section is cut away and a spool piece is fabricated to reconnect the open pipeline ends. Similarly, it is sometimes required that a spool piece be fabricated to connect an underwater pipeline with a riser flow pipe that extends from the ocean floor to the deck of an offshore platform or between two pipe sections laying on the ocean floor.
In the above cases, the pipeline sections will probably not have a coaxial relationship, but will instead have different orientations. Thus, when attempting to fabricate a spool to couple the underwater pipeline sections together, it is necessary to have precise information describing the spatial relation of the ends of the two pipelines. Without an accurately constructed spool, the spool and pipeline end flanges will not properly fit together, requiring that the ends of the pipeline sections be forced into proper alignment or a long bolting technique be used, both of which lead to stress in the pipe sections and in the make-up joints, and also requiring long intervals of diver time to complete the connection.
In making underwater tie-ins in connection with either pipeline joinder or repair or riser flow pipe hook-ups, it is necessary to proceed through four distinct phases of the overall operation. These phases are: pipeline preparation, determination of pipeline orientation, fabrication of a connecting spool, and installation of the spool.
The phase of pipeline preparation involves preparation of the underwater pipeline ends to be joined. The underwater pipeline ends are cut to define a face perpendicular to the axis of the pipeline so that the exposed ends define a circle. It will be appreciated that if the pipeline ends are cut at an angle to the axis of the pipeline, the exposed end will define an ellipse thereby making spool fabrication very difficult. The pipeline ends are also cut to afford enough space between the ends that the connecting spool will not include any severe bends which could present a significant resistance to flow. Hence, in pipeline joinder operations, the ends to be joined are in planes perpendicular to the pipeline axes. However, most frequently, these planes are at an angle to one another and the axes of the pipeline will not often intersect resulting in the necessity to accurately fabricate a spool in order to effect joinder without undue stressing or manipulation of the pipeline ends.
Determination of the orientation of the pipeline ends becomes the next step in the joinder operation. One prior art technique for making underwater tie-ins is that discussed in the article "Flanged Spool Connects Subsea Pipeline Ends", THE OIL AND GAS JOURNAL, Feb. 3, 1974, pages 92, 93 and 96. In that technique, templates or flanges are affixed to each pipeline end and are welded together with rods extending between the pipeline ends. The resulting structure then provides a model or form from which the orientation of the pipeline ends may be determined and from which the spool may be fabricated. Creating the form, however, involves significant diver time. In addition, stresses introduced in the form by welding and raising the form to the surface can produce errors when the spool is fabricated from measurements taken from the form. In addition, there is no accurate technique for inserting the spool between the pipeline ends without excessive diver time.
In copending U.S. application Ser. No. 609,196 entitled "Apparatus for Determining the Spatial Relation Between Two Misaligned Members" filed Sept. 2, 1975 in the name of Maurice P. Lebourg, there is disclosed apparatus for obtaining accurate information concerning the orientation of underwater pipeline ends, and for fabricating a spool for insertion therebetween.
The method and apparatus of this invention is addressed to the final step in the joinder operation, i.e., inserting a spool between the pipeline ends. It may be used to install a spool which has been constructed using spatial orientation information obtained from the welded template technique or using an apparatus of the type disclosed in the above-identified copending application.